Bank segment - Repurchase Agreements (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Offsetting Liabilities [Line Items]
Gross amount of recognized liabilities	$ 88	$ 93
Gross amount offset in the Balance Sheet	0	0
Securities sold under agreements to repurchase	88	93
Securities sold under agreements to repurchase collateral, financial instruments	120	116
Securities sold under agreements to repurchase, cash collateral pledged	0	0
Financial Institution
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	0	0
Securities sold under agreements to repurchase collateral, financial instruments	0	0
Securities sold under agreements to repurchase, cash collateral pledged	0	0
Government Entities
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	0	14
Securities sold under agreements to repurchase collateral, financial instruments	0	15
Securities sold under agreements to repurchase, cash collateral pledged	0	0
Commercial account holders
Offsetting Liabilities [Line Items]
Securities sold under agreements to repurchase	88	79
Securities sold under agreements to repurchase collateral, financial instruments	120	101
Securities sold under agreements to repurchase, cash collateral pledged	$ 0	$ 0